1. Field of the Invention
The present invention is directed to an apparatus and process for measuring the changes in the level of liquid stored in storage tanks. More particularly, the present invention is directed to an apparatus and process for measuring leakage of liquids, such as hydrocarbon fuels, out of underground storage tanks in a manner which compensates for density changes of the stored liquid which may occur during the measurement due to changes in the temperature of the stored liquid.
2. Brief Description of the Prior Art
Leakage to the surrounding environment of hydrocarbon fuels from underground storage tanks normally associated with filling stations represents a significant environmental and fire hazard, especially where a relatively large number of filling stations are located in urban or other environmentally sensitive areas. Leakage from underground storage tanks of liquid chemicals or flammable liquids other than hydrocarbon fuels likewise create significant environmental and fire hazards. For this reason, most jurisdictions in the United States require periodic inspection of storage tanks of filling stations, and usually set the permissible maximum limit of leakage from an entire tank at approximately 0.05 gallon of hydrocarbon fuel (gasoline or diesel fuel) per hour.
A well established and relatively simple procedure of the prior art for inspecting storage tanks of filling stations for leakage includes the steps of adding fuel to the tank until the liquid level is disposed within the filling pipe of the tank, marking or noting the liquid level, and thereafter adding an additional measured amount (for example, 0.5 gallon) of fuel. The rise of the level of the liquid in the usually cylindrical fill pipe is then measured in inches or centimeters. Thereafter, the tank is kept undisturbed, that is, fuel is not dispensed from the tank for several hours, usually at least for four hours. At the end of the prescribed time period, the level of liquid in the fill pipe is measured again, and the leakage rate is calculated from the results of the above-summarized measurements. Occasionally, instead of fuel leaking out of the tank, ambient ground water leaks into the tank. Theoretically, the above-described process is capable of detecting this type of tank defect also.
Serious disadvantages of the above-described prior art measuring or inspecting process include the following.
Measuring of liquid levels by a metering rod, or like gauge, is inherently inaccurate enough so that it can be conducted with any degree of reliability only in the relatively narrow cylindrical fill pipe. Moreover, because the typical storage tank is a horizontally disposed cylinder which has gradually narrowing curved upper walls, no meaningful "gallon per hour" figure is calculated from simple linear liquid level measurements, except in the vertically disposed cylindrical fill pipe. Filling the tank to such a high level, however, increases the hydrostatic pressure in the storage tank above the average or usual pressure, to the extent that the underground structure of the tank is subjected to a potentially significant expansion resulting in a gradual change in the volume of the tank. The extent of this extension of structure and the resulting volume change is, however, unpredictable because it is strongly influenced by the conditions of the soil wherein the storage tank is buried. Experience has shown that the error, which may result from a change in tank volume caused by the increased hydrostatic pressure, may very well be in the magnitude of 0.05 gallon per hour. Such an error may, of course, cause the results of the entire leakage measurement to be totally erroneous to the point where impermissible leakage is detected where it does not exist. Alternatively, such an error may totally mask impermissible leakage of ground water into the tank.
It is apparent from the foregoing that a process and appropriate implementing apparatus are needed which measure liquid levels in storage tanks without requiring the tanks to be filled to the brim for the measurements. Moreover, changes in the level of the liquid must be monitored with much greater accuracy than what is attainable by a simple measuring rod, especially when the measurement is not conducted with the liquid levels being in the cylindrical fill pipe.
In partial fulfillment of these objectives instruments were designed relatively recently for use in underground storage tanks of filling stations which measure liquid levels with much greater accuracy than a simple metering rod. U.S. Pat. Nos. 4,397,183 and 4,373,815 describe, for example, apparatus wherein a signal emitter emits laser or infrared light which is, in turn, reflected from a float tracking the liquid level in the tank. The reflected signal is received in a receiver and is utilized to measure the position of the float with a relatively large degree of accuracy.
U.S. Pat. Nos. 4,474,054 and 4,561,291 disclose apparatus wherein a slope tube is mounted to the fill pipe of a storage tank, the level of fuel in the tank is maintained in the fill pipe, and gas is slowly bubbled into the fill pipe in such a manner that the gas pressure maintains a liquid level in the slope tube. As the liquid level changes in the fill pipe due to leakage, the pressure of the gas bubbling up through the liquid in the fill pipe also changes, and the resulting change is detected in the slope tube.
A publication by the U.S. Environmental Protection Agency, titled UNDERGROUND TANK LEAK DETECTION METHOD: A STATE-OF-THE-ART REVIEW, dated January, 1986, reviews still other types of apparatus and processes for detecting leaks in underground storage tanks.
Although the above-mentioned and relatively recently developed prior art solves some of the problems associated with detection of leaks from underground fuel tanks, significant problems remain unsolved in this connection. For example, still many of the recently developed prior art methods require filling the tank until the liquid level reaches the fill tube. Moreover, the temperature of the fuel in the tank frequently changes enough to render the entire measurement erroneous, because the density and hence the volume of the fuel changes with temperature.
The prior art has attempted to compensate for these effects of temperature changes by monitoring the temperature of the fuel in the tank and then calculating an adjustment for the resulting density changes. An alternative method of the prior art for compensating the effects of temperature changes during the leakage measurements comprise monitoring the fuel level in a closed container submerged in the tank. This alternative method utilizes the principle that level changes in the closed container are necessarily due to temperature induced density changes only, and cannot be attributed to leakage. The measured level change in the closed container is then used as a compensating factor to be deducted from, or added to, level changes in the storage tank. This alternative method of the prior art, however, fails to take into consideration the fact that level changes in a vertically disposed cylindrical container are, generally speaking, not identical to the level changes caused by identical density changes in the storage tank. This is because the storage tank is a horizontally disposed cylinder having curved walls which result in gradually changing vertical cross-sections.
It is apparent, in light of the foregoing, that apparatus and processes are needed in the art which are capable of accurately measuring leakage of liquid in and out of underground storage tanks, and which are capable of compensating for temperature-induced density changes of the liquid. The present invention provides such apparatus and processes.